How is numerosity estimation affected by additional structural information in visual displays? Two experiments investigated if the linking of dots by line segments, thereby forming clusters of polygons, leads to an underestimation effect similar to that observed in classical experiments on clustering by spatial proximity. Our findings confirmed such an underestimation effect for non-spatial clustering. In addition, the relative magnitude of the underestimation increased along with the number of objects and with cluster size. Finally, we observed that the presence of an equivalent number of line segments as in the previous condition, however unaligned with the dots, reduced the underestimation effect to a constant relative magnitude